This invention relates to cold water-soluble polymer particles or particulate product having excellent wettability with water and capable of quickly dissolving in water without forming undissolved powder (undissolved lump of flour) and also to a method for preparing such particles.
Water-soluble polymers have been hitherto employed in various fields of cosmetics or building materials using cements. Water-soluble cellulose ethers are particularly useful as a water-soluble polymer. They are prepared by providing pulp as a starting material, subjecting to acetylation reaction in an alkaline solution, followed by etherification reaction, and washing in hot water or in an organic solvent containing water. Thereafter, the resulting purified water-soluble cellulose is isolated from the solution by filtration, dried with hot air and ground to pieces.
The water-soluble polymer particles such as water-soluble cellulose ethers obtained according to the above procedure have the following problems.
The particles of the water-soluble polymer such as the water-soluble cellulose ether prepared as above are poor in solubility and requires a long time for dissolution since the surface portion of each water-soluble polymer particle is readily dissolved in water to form a strongly tacky film at the surface thereof upon dissolution, whereby the polymer particles adhere each other by stirring upon dissolution to form large undissolved lump of flour. To prevent the formation of undissolved lumps of flours, for one type of water-soluble polymers such as methyl cellulose, hydroxypropylmethyl cellulose, hydroxyethylmethyl cellulose and the like, a complicated procedure of slowly dispersing the cellulose ether in hot water provided beforehand and gently cooling for dissolution has been adopted so as to make use of their properties of not being dissolved in hot water of 90° C. or over. For an other type of water-soluble polymers such as polyethylene oxide, sodium polyacrylate and the like, a complicated procedure has also been adopted wherein dispersion is carried out in water especially in a high-speed agitating device so that undissolved powder is not left, followed by dissolution under agitation.
In order to avoid such problems described above, JP-B 48-6622 has proposed a method of preventing undissolved powder from being left in cold water, in which 0.01 to 15 parts by weight of at least one plasticizer selected from ether, ester and amine plasticizers is added to water-soluble cellulose ether powder, uniformly mixed and dried, and crushed to coarse particles. With the coarse particles obtained by this method, however, the plasticizer is mixed with a wetting agent to obtain granules to lumps, which are dried and subsequently ground to pieces. In this way, such a dense product that has been eventually obtained by mixing and densifying during the mixing and drying steps is ground, thus resulting in granules having fibrous portions and fine powder to granules having fibrous fractions on the surfaces thereof to cause undissolved powder to be formed in cold water. To prevent this, a great amount of water is charged when the powder is mixed after addition of a plasticizer so that an apparent bulk density is increased, after which the resulting mixture is dried and pulverized. This may cause a problem in that the bulk density of the resulting granules becomes too high and thus, it takes a long time for dissolution.
In JP-A 6-166026, there is proposed a granulation method wherein particles or granules obtained by use of a specific type of granulator are dried to provide spherical granules which have a reduced amount of fibrous matters, are substantially uniform and close to a sphere in shape, and are almost constant with respect to particle size. Nevertheless, this method cannot prepare granules that can prevent occurrence of undissolved powder in cold water. This is because wettability of water-soluble cellulose ethers is so poor that infiltration of water into the thus prepared, individual granules becomes inadequate.
On the other hand, with water-soluble cellulose ethers, an attempt has been made in JP-B 42-6674, in which a crosslinking agent such as a dialdehyde is applied onto the powder surfaces so as not cause undissolved powder to be formed in cold water, and after dispersion of the powder, the crosslinked structure is destroyed by addition of an alkali component, thereby facilitating solubility. Moreover, in JP-A 2000-63565, there has been proposed preparation of particles which are obtained by adding a crosslinking agent, an acid and a wetting agent to a water-soluble cellulose ether and wherein the content of particles passing through a 30 mesh sieve is not higher than 30% by weight and the residual amount on a 200 mesh sieve is not higher than 30% by weight. Such particles are free of powder dust and excellent in wettability with water, and can be dissolved within a short time without forming undissolved powder in cold water. In this connection, however, monoaldehydes such as formaldehyde, butylaldehyde, glycerine aldehyde and the like, and dialdehydes such as glyoxal, acetaldehyde, terephthalaldehyde and the like exhibit mutagenic properties, and there is concern about an environmental and hygienic problem as would be caused by the mutagenic properties. On the other hand, when crosslinking agents of expensive silicon compounds including alkylalkoxysilanes such as tetramethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane and the like are added, compatibility may become worsened in some cases, with the attendant problem that use can not be made for cosmetics or agents for suspension polymerization.